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Subject: Re: [PATCH AUTOSEL 4.19 087/209] ARM: dts: da850-lego-ev3: slow down
 A/DC as much as possible
To:     Sasha Levin <sashal@kernel.org>, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org
Cc:     Sekhar Nori <nsekhar@ti.com>, devicetree@vger.kernel.org
References: <20191113015025.9685-1-sashal@kernel.org>
 <20191113015025.9685-87-sashal@kernel.org>
From:   David Lechner <david@lechnology.com>
Message-ID: <96753746-79ae-3db2-c6d8-e4d5615c0354@lechnology.com>
Date:   Wed, 13 Nov 2019 10:10:30 -0600
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On 11/12/19 7:48 PM, Sasha Levin wrote:
> From: David Lechner <david@lechnology.com>
> 
> [ Upstream commit aea4762fb46e048c059ff49565ee33da07c8aeb3 ]
> 
> Due to the electrical design of the A/DC circuits on LEGO MINDSTORMS EV3,
> if we are reading analog values as fast as possible (i.e. using DMA to
> service the SPI) the A/DC chip will read incorrect values - as much as
> 0.1V off when the SPI is running at 10MHz. (This has to do with the
> capacitor charge time when channels are muxed in the A/DC.)
> 
> This patch slows down the SPI as much as possible (if CPU is at 456MHz,
> SPI runs at 1/2 of that, so 228MHz and has a max prescalar of 256, so
> we could get ~891kHz, but we're just rounding it to 1MHz). We also use
> the max allowable value for WDELAY to slow things down even more.
> 
> These changes reduce the error of the analog values to about 5mV, which
> is tolerable.
> 
> Commits a3762b13a596 ("spi: spi-davinci: Add support for SPI_CS_WORD")
> and e2540da86ef8 ("iio: adc: ti-ads7950: use SPI_CS_WORD to reduce
> CPU usage") introduce changes that allow DMA transfers to be used, so
> this slow down is needed now.
> 

I doubt that the commits above would be backported, so it does not
make sense to backport this patch.